Every year, emissions regulations are becoming stricter with regard to substances exhausted from diesel engines, such as particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC). As these regulations become stricter, it is becoming difficult to meet the stipulated values by engine improvement alone. Consequently, technology is being adopted wherein an exhaust gas post-treatment device is installed in the exhaust passage of the engine to reduce these substances exhausted from the engine.
In such circumstances, various research and proposals have been made regarding NOx catalysts for the elimination by reduction of NOx from the exhaust gas of internal combustion engines, such as diesel engines and some gasoline engines, and various combustion devices. One of these catalysts is a NOx occlusion-reduction catalyst that acts as a NOx-lowering catalyst for diesel engines. With the use of this NOx occlusion-reduction catalyst, NOx in exhaust gas can be effectively purified.
This NOx occlusion-reduction catalyst is constructed as a monolith honeycomb that forms a large number of polygonal cells on a support body of structural material formed using cordierite, silicon carbide (SiC), or stainless steel. Provided on the walls of these cells is a porous catalyst coat layer formed using alumina (Al2O3) that acts as the catalyst-carrying layer. The catalyst coat layer carries on its surface a noble metal catalyst having oxidation functions, as well as a NOx occluding agent (NOx occluding substance; NOx occluder; NOx absorber) having NOx-occluding functions. The noble metal catalyst is formed using platinum (Pt) or a similar metal. The NOx occluding agent is formed from several among the following: alkali metals, such as potassium (K), sodium (Na), lithium (Li), and cesium (Cs); alkali earth metals, such as barium (Ba) and calcium (Ca); and rare earth metals, such as lanthanum (La) and yttrium (Y). In so doing, the two functions of NOx occlusion and NOx release/purification are realized depending on the oxygen concentration in the exhaust gas.
This NOx occlusion-reduction catalyst occludes NOx to the NOx occluding agent during regular engine operation. When the occluding ability of the NOx occlusion-reduction catalyst nears saturation, the air-fuel ratio of exhaust gas flowing into the NOx occlusion-reduction catalyst is brought to a rich air-fuel state at an appropriate time, thereby causing occluded NOx to be released. Additionally, the released NOx is reduced by the three-way function of the noble metal catalyst.
More specifically, when the air-fuel ratio of exhaust gas is in a lean air-fuel state, such as the oxygen (O2)-containing exhaust gas of regular diesel engines and lean combustion gasoline engines, nitrogen monoxide (NO) exhausted from the engine is oxidized to nitrogen dioxide (NO2) by the oxidation catalyst functions of the noble metal catalyst using the oxygen present in the exhaust gas. Subsequently, this nitrogen dioxide is occluded in the form of a salt such as nitrate by a NOx occluding agent such as barium, thus purifying the NOx.
However, if this state is continued as-is, the NOx occluding agent having NOx occluding ability will be entirely transformed into nitrate, and NOx occlusion functions will be lost. Consequently, exhaust gas with an exceedingly high concentration of fuel (rich spike gas) is created by changing the operational parameters of the engine or by injecting fuel into the exhaust passage, and this rich spike gas is then sent to the catalyst. The rich spike gas is a high-temperature exhaust gas having a high concentration of carbon monoxide (CO), and wherein oxygen is not present.
When the exhaust gas is thus brought to a rich air-fuel state wherein oxygen is not present, wherein there is a high concentration of carbon monoxide, and wherein the exhaust gas temperature has been raised, the nitrate formed by the occlusion of NOx releases nitrogen dioxide and returns to the original barium or other catalyst substance. Since oxygen is not present in the exhaust gas, the released nitrogen dioxide is reduced to nitrogen (N2) and purified by the oxidation functions of the supported noble metal, using the carbon monoxide, hydrocarbons (HC), and hydrogen (H2) present in the exhaust gas as reducing agents.
For this reason, a NOx regeneration operation is conducted in NOx purification systems provided with a NOx occlusion-reduction catalyst, in order to make the NOx occlusion-reduction catalyst function effectively. In this NOx regeneration operation, when NOx occluding ability nears saturation, the amount of fuel in the exhaust gas is raised beyond the theoretical air-fuel ratio, thereby bringing the exhaust gas to a rich air-fuel state in order to cause occluded NOx to be released and regenerate the catalyst. In so doing, the oxygen concentration of inflowing exhaust gas is decreased, and exhaust gas of a reducing composition is supplied to the catalyst. By conducting this rich control for recovering NOx occluding ability, the NOx absorbed by the NOx occluding agent is released. The released NOx is then reduced by the noble metal catalyst.
In addition, a rich control for recovering NOx reducing ability is conducted for NOx direct reduction type catalysts, a type of catalyst different from NOx occlusion-reduction catalysts. In a NOx direct reduction type catalyst, NOx is directly reduced in the lean state, while the catalyst is regenerated in the rich state. In addition, a rich control is also conducted in continuous regeneration DPF devices in order eliminate by combustion soot (PM) trapped at the filter.
In this rich control, it is necessary to supply the sufficient required amounts of reducing agent and uncombusted fuel, which is used as fuel for raising the exhaust gas temperature. In a diesel engine, in order to generate reducing exhaust gas, reducing agent HC is supplied by post-injection (after-injection) as part of an in-cylinder fuel injection. If the rich state is realized using only a fuel-related rich control wherein such fuel is added, fuel efficiency worsens, and drivability also worsens due to torque variations. For this reason, an intake-related rich control is used in conjunction with the above. In this intake-related rich control, air intake is throttled using an intake throttle valve, while in addition an exhaust gas recirculation (“EGR”) valve is opened and a large amount of EGR gas is supplied. In so doing, the amount of intake air is reduced and the amount of inspired oxygen is lessened.
However, conducting such an intake-related rich control involves the following problem. During the rich control period wherein the EGR ratio is high, if exhaust gas containing highly dense uncombusted fuel (HC) is made to pass through the EGR passage, then uncombusted fuel and soot adhere to the EGR cooler and EGR valve of the EGR passage, as well as to the intake port and intake valve. The EGR cooler farthest upstream in particular becomes clogged in a short amount of time. For this reason, the intake-related rich control becomes impeded, and a sufficient rich control becomes unavailable.
As a countermeasure for such clogging of the EGR cooler, exhaust gas recirculation devices for diesel engines have been proposed such as that disclosed in Japanese Patent Application Kokai Publication No. H6-66208, for example, being an exhaust gas recirculation device for a diesel engine provided with a particulate trap and an oxidation catalyst layer downstream thereto in the EGR gas recirculation passage. With this device, not only soot but also uncombusted hydrocarbons in the EGR gas recirculation passage are eliminated, and the adherence and accumulation of these substances at the intake port and intake valve is prevented.
In addition, exhaust gas recirculation devices for internal combustion engines have been proposed such as that disclosed in Japanese Patent Application Kokai Publication No. 2005-16390, wherein an electric heater, particulate filter, EGR cooler, and EGR valve are disposed in that order from the upstream side of the recirculation passage. The state (on/off) of current flow to the electric heater is then switched on the basis of the temperature downstream to the particulate filter. With this device, clogging of the cooling device in the recirculation passage (EGR passage) due to particulates is suppressed. The particulate filter is also made to function effectively and without blockage, even under conditions wherein the temperature of exhaust (EGR gas) flowing into the EGR passage is low.
These devices have the following problem. Since soot (PM) is trapped by a particulate filter (or trap) and uncombusted hydrocarbons are eliminated by oxidation using an oxidation catalyst, it is necessary to regenerate the particulate filter from soot accumulation. This soot occurs not only during the rich control, but also during the lean control, and thus the particulate filter regeneration control must be conducted frequently, which leads to a more complicated control and worsened fuel efficiency.
Meanwhile, through experiment and other means, the present inventors have discovered that uncombusted fuel acts as a binder that causes soot, together with the uncombusted fuel, to adhere to the EGR cooler and other areas in the EGR passage. Furthermore, the inventors have discovered that there is hardly any adherence to the EGR cooler, EGR valve, intake port, intake valve, and other areas in the case of only dry soot that does not contain uncombusted fuel. Such dry soot reaches the interior of the cylinders and is purified by combustion.